reaction

noun

a reverse movement or tendency; an action in a reverse direction or manner.

movement in the direction of political conservatism or extreme rightism.

action in response to some influence, event, etc.: the nation's reaction to the president's speech.

Physiology. action in response to a stimulus, as of the system or of a nerve, muscle, etc.

Medicine/Medical.

the action caused by the resistance to another action.

a return to the opposite physical condition, as after shock, exhaustion, or chill.

Bacteriology, Immunology. the specific cellular response to foreign matter, as in testing for allergies.

Chemistry. the reciprocal action of chemical agents upon each other; chemical change.

Also called nuclear reaction.Physics. a process in which a nucleus that is bombarded by a photon, particle, or other nucleus, emits a nucleon, alpha particle, or the like, without a significant change in its atomic weight.

Mechanics. the instantaneous response of a system to an applied force, manifested as the exertion of a force equal in magnitude but opposite in direction to the applied force.

usage

Reaction is used to refer both to an instant response (her reaction was one of amazement) and to a considered response in the form of a statement (the Minister gave his reaction to the court's decision). Some people think this second use is incorrect

Originally scientific; physiological sense is attested from 1805; psychological sense first recorded 1887; general sense of "action or feeling in response" (to a statement, event, etc.) is recorded from 1914. Reaction time, "time elapsing between the action of an external stimulus and the giving of a signal in reply," attested by 1874.

reaction

[ rē-ăk′shən ]

A rearrangement of the atoms or molecules of two or more substances that come into contact with each other, resulting in the formation of one or more new substances. Chemical reactions are caused by electrons of one substance interacting with those of another. The reaction of an acid with a base, for example, results in the creation of a salt and water. Some, but not all, reactions can be reversed.

See nuclear reaction.

An action that results directly from or counteracts another action, especially the change in a body's motion as a result of a force applied to it. Some reactions counteract forces and are not readily apparent. When an object rests on a surface, such as a table, for example, the downward force it applies to the surface is counteracted by an equal but upwards force, or reaction, applied by the surface. See more at Newton's laws of motion.

A response to a stimulus, such as a reflex.

The response of cells or tissues to an antigen, as in a test for immunization.